The cold setting products manufactured from fly ash comprises materials in form of brick, block, tiles, plate, concrete or monolithic, aggregates etc, which possess sufficient mechanical strength, low porosity and water absorption, heat and acid resistance properties. These products of fly ash are suitable for use in road building and other constructional activities including construction of heat and acid resistance structures. The materials made up by chemical activation of fly ash along with other additives are also useable for encapsulation and fixation of various toxic constituents of polluting solid wastes in making rock forming bodies for safe disposal.
Coal fired thermal power plants in the process of electricity production generate burnt mineral matters of coal known as ash. These are particulate materials and collected from the boiler exhaust gas and grate in form of fly ash as major fraction and bottom ash in smaller amount. All together it is mixed and disposed as fly ash. Generation of ash of coal fired thermal power plant is enormous and mostly it remains unused as a waste material. Gradual accumulation of thermal plant fly ash overloads in disposal problems and pollution of air, water and soil of the surrounding environment. Utilization of fly ash in gainful manner is a serious concern for sustainable growth of thermal plant and to save the associated environment pollution. Many research activities and process developments have been made for the use of fly ash in construction, agriculture, land and mine filling etc. A considerable work and many process developments also have been carried out for the utilization of fly ash as a raw material in manufacture of building material products like brick, block, and concrete, hydraulic cements etc. Utilization of fly ash construction sector is a greater important to save natural occurring soil, clay and aggregates for the protection of environment. Manufacturing of building bricks from clay or alluvial soil by firing in brick kilns is an age old and traditional practice. Utilization of fly ash in manufacturing of building brick and block like products by replacing clay has been given more focus for its wider application in all sectors of society.
Fly ash, a waste of thermal power plant, is a different type of non traditional material as compared to clay or alluvial soil. Many processes have been developed on manufacturing of fly ash based building brick and block like products. It is mostly used as a raw material in different proportions for filler, matrix and bond forming agent, chemical admixture in association with other additives to develop binding strength in manufacture of strong and acceptable quality building brick or block and other construction materials. Different methods and manufacturing steps such as firing to form oxide matrix bonding by thermal effect, hydro-thermal and hydration reactions to form hydroxide matrix bonding are well known and already in practice to use fly ash mix materials in production of building brick, block and construction related products.
So far sufficient R & D efforts have been put into study on manufacturing of fly ash based building brick and block like products. Reference may be made to the following disclosures: Firing methods at different temperature in manufacturing of building brick and related construction materials from fly ash has been disclosed. Fly ash is used with other additives and also plasticizing material in various proportions to form green moulds of brick or block shape products and it is subjected for drying and firing to develop strength for use as a construction material. The process involves various conventional steps of mixing, molding, drying/curing and firing. Fly ash in association with other additives develops oxide bonding matrix by thermal reaction which imparts strength to the products suitable for building construction use. Different workers have revealed the process on the use of various additives and methods of mixing, molding, drying of fly ash based materials in manufacture of fired building products.
Smith; Robert H (U.S. Pat. No. 4,120,735) describes the manufacturing of fired bricks using coal fly ash with 50% weight of other non-ferrous and municipal incinerator residue and sodium silicate binder for preparation of green articles of brick or block, curing and drying of green articles 12 hours to 5 days at low temperature of below 100° C. and then firing in between 1000° C. to 1300° C. for 30 minutes to 6 hours depending up on the mix proportion. The fly ash based fired building products produced by this method develops strength up to 30 MPa and 5 to 20% water absorption.
Riddle; Mitchell S (U.S. Pat. No. 5,405,441) also explains the manufacturing of building construction brick and other material from fly ash and other hazardous wastes by firing method. It discloses the use of pozzolanic fly ash with the additives of cement, lime, gypsum plaster, polymers, resins, volcanic ash, clay, aggregate, hazardous wastes includes asbestos, contaminated soil, pesticides, medical and municipal incinerator ash, PCB contaminants, radioactive waste, heavy metal and gasification ash in production of waste encapsulated solid brick and block like products useful as a construction material adopting firing for sintering at 1400° C. and coating of exterior surface by liquefying fly ash mixture.
U.S. Pat. No. 5,665,290 describes a manufacturing process of making fired building brick from burnt materials of brown coal. The process discloses the use of various additive materials like ground glass, quartz, lime with brown coal ash in presence of water to improve the pozzolanic reaction and stability in drying the molded articles suitable for preparation of fired building products.
Kayali, Obada; et al. (U.S. Patent No. 20050109242) describes manufacturing of fly ash matrix bonded sintered fly ash brick as a replacement of conventional fired clay brick. It discloses the use of both type of class-F and —C fly ash blended with plasticizer carboxymethyl cellulose or calcium chloride and water in preparation of green articles, curing of the articles at 30 to 80° C. at 20% to 60% relative humidity for 12 hours to 5 days and firing of the article in between 1000 to 1300° C. in manufacture of building material brick and block like products. Use of high lime fly ash like class-C improves the rate of solidification at green stage and reduces curing period and firing time of the articles. The bonding material is mostly hydrates of calcium silicate and alumino-silicate at the green stage and on firing it develops oxide structures as bonding matrix of the fly ash. The bricks produced by this process exhibits crushing strength more than 30 MPa, modulus of rupture more than 5 MPa and 5 to 20% water absorption and the product quality is superior than conventional fired clay brick.
Hydro-thermal process has also been used to produce building material products. This is a process of accelerated curing under temperature and steam pressure to enhance the formation of binding matrix in development of strength of the building material product in short time. This method of hydro-thermal treatment has already been reported in manufacture of calcium silicate and sand-lime bricks using the mixtures of sand/siliceous materials and lime. Lime (CaO) present in the mixture reacts with silica and siliceous constituents under hydro-thermal condition and forms crystalline structures of hydrated calcium silicate phases as the binding matrix in the product. Utilization of fly ash as a substitute for sand or siliceous material in manufacture of Sand-Fly ash-Brick has been disclosed in the report “Material Research Standards, USA, 1964 and Research Association of Calc-Sand stone, Germany”. Under hydro-thermal reaction the fly ash base mixture with lime develops multi phases of calcium base hydrated mineral structures which impart bonding of the particulates and strength to the product. The bonding phases in the fly ash based materials are mostly hydrated crystalline phases of calcium-aluminum-silicate and calcium silicates. Hydro-thermally developed binding phases in manufacture of fly ash based building materials has been disclosed by many workers.
U.S. Pat. No. 3,501,323 discloses the hydro-thermal processes for manufacture of building, structural and paving products based up on blending extremely fine siliceous with calciferous materials The molds of the mixture prepared in presence of water by compaction pressure is hydro-thermally treated at a temperature of about 250° C. for a time a time of 30 minutes to form calcium-silicate binding matrix for strength in the product.
German Patent application DE-A-3321899 describes the use of coal fired power plant ash, slacked lime and water in preparation of brick-form elements after which the bricks are hardened with the use of steam pressure at 14-16 bar and temperature of between 180 to 220° C.
U.S. Pat. No. 4,683,006 discloses in production of construction aggregates using the mixtures of pulverized fly ash with lime. The shaped article prepared from the mixture is cured in a moisture controlled atmosphere in the temperature range of 35° C. to 80° C. The strength of the shaped articles is attributed by the formation of hydrated crystallites of ettringite and amorphous calcium silicates, calcium aluminite and calcium aluminum silicates.
Loggers; Hendrik (U.S. Pat. No. 4,780,144) discloses some improvement in the process steps in making fly ash based building articles under hydrothermal reaction at low pressure and temperature. It describes the use of fly ash preferably calcium sulfate containing ash, slacked lime, quartz flour and water in formulation of granulated particles by hardening at 100° C. temperature in atmospheric pressure and then mixing of these hardened particles as a replacement of coarse materials with or without gravel, sand, B.F. Slag, gypsum and hematitic rock in formulation of useable building elements by hardening in steam chamber under atmospheric pressure for 8 hours. Formulation of dimensional stable building material article is made under the influence of vibration, shocking, pressing or extruding. Strength development under the hydro-thermal curing in the brick/block like elements is in between 12 to 28 N/mm2.
Strabala; William W (U.S. Pat. No. 5,534,058) discloses manufacturing of lightweight, good strength and fire resistant structural products such as brick, panels and studs etc using fly ash with the additives of cellulose based (wood, paper pulp) materials and adhesive (polyvinyl acetate) binders. The molded articles prepared by applying pressure are cured in steam (water vapor) at a temperature up to 150° F. for a period of about 28 days to maximize the strength and hardness suitable for building material use.
Seike, et al. (U.S. Pat. No. 5,584,895) describes the method of hydro-thermal treatment in manufacture of fly ash mix building materials. Fly ash based molded articles prepared with water and lime bearing materials like calcium oxide, calcium hydroxide is cured at 30 to 90° C. temperature for 2 to 5 hours and then the cured articles are treated for hydro-thermal reaction under saturated steam vapor pressure between 120 to 250° C. for hardening. Fly ash with reaction of lime under this condition mainly produces tobermorite, a hydrated phase of calcium and silica (C—S—H) as the binding matrix to impart strength to the extent of 400 kg/cm2 in the product suitable for making concrete like articles. It also discloses the use of blowing agent to make lightweight and hydroxides and carbonate of alkali metals and ammonia to improve strength and freeze-thaw resistance of fly ash based products.
Weyand, et al. (U.S. Pat. No. 6,068,803) discloses the hydro-thermal process in manufacture of building brick and block like products from fly ash. The molded articles prepared from the mixtures of fly ash and lime are hydro-thermally cured in the saturated steam temperature of between 180-225° C. under pressure 150 psig to 350 psig for 4 to 8 hours in the autoclave. Strength in the product is achieved by hydro-thermally grown interlocking matrix of calcium silicate based Tobermorite (Ca5Si6O17.5H2O) crystalline phases.
Hydraulic hardening by curing under atmospheric condition is also in use for manufacture of fly ash based building construction materials. The pozzolanic property of the fly ash is being considered in this process to develop cementitious binding matrix under natural hydration in presence of additives mostly lime bearing materials. The hydration reaction on formation of bonding matrix under natural condition is slow as compared to the hydro-thermal treated products. Many findings of fly ash application as a cementitious filler and pozzolanic material in manufacturing of hydraulic cements, concrete and building products have been disclosed. Processes on manufacturing of hydraulic setting building bricks and blocks from fly ash by natural curing have been described.
U.S. Pat. Nos. 4,397,801, 5,350,549 and 5,211,750 reveal the use of different additives such as spent fluid bed combustion residue containing limestone, oxidic sulphur, with or with out port land cement with fly ash to form hydraulic bonding matrix by curing for 28 days in atmospheric condition in manufacture of building brick and block.
U.S. Pat. Nos. 5,358,760, 5,362,319, 5,366,548 and 5,374,307 disclose the manufacture of cementitiously bonded construction products of fly ash brick and block by curing in atmospheric condition using various additives like lime, clay, port land cement, gypsum, calcium carbonate, plaster of paris and oxidants.
U.S. Pat. No. 5,154,771 discloses the use of fly ash with port land cement, polysaccharide molding aid and reinforcing fiber and water in manufacture of cold setting building products. U.S. Pat. No. 5,366,548 describes the manufacturing of construction brick and blocks using class-c fly ash with kiln dust by applying high pressure during molding and then atmospheric curing. U.S. Pat. Nos. 3,625,723 and 4,659,385 describe manufacture of cellular light weight construction product from fly ash with the combination of aluminum phosphate solution, calcium silicate, aluminum hydroxide, air entrainer and foaming agent etc as the bonding matrix.
Development of cementation matrix for hardening the substances primarily calcium base hydrated structures of similar characteristics of hydrated Portland cement by hydro-thermal reaction under temperature and pressure and natural hydration under atmospheric condition have been reported in the manufacture of fly ash building bricks and related products.
Further many other binding materials other than Portland cement have been explored to use in the construction industry and solidification of waste materials. Some efforts have been made on the development of alkaline base inorganic binders for their application in solidification of waste materials and also in making structural products. The said inorganic binders have been prepared by admixture of various constituents which forms hydroxylated structures and develops binding property.
U.S. Pat. No. 5,820,668, Comrie discloses preparation of inorganic binders of two different compositions based upon the admixtures of (sodium oxide, tricalcium silicate, dicalcium silicate, calcium aluminate, aluminium oxide, ferric oxide, sulphur trioxide) and (fumed silica, anhydrous aluminium silicate, silicic acid, sodium or potassium salts). The inorganic binders are curable in saline, corrosive and acidic environmental conditions. These binders are used as a replacement of cement in agglomeration, briquette form, of metallurgical wastes like coke breeze, slag, mill scale and flue dust for recycling into the furnace.
U.S. Pat. No. 5,194,091, Laney discloses a method of making self hardening inorganic binder based upon the admixture of alkali metal silicate, calcium meta-silicate (wollastonite) as strengthening agent, hydrous aluminium silicate (clay type) as thickening agent, sodium fluorosilicate as setting agent, calcium/magnesium chloride and zinc oxide applicable in formulation of bonding matrix in strengthening of gypsum base construction products.
Loney in other work U.S. Pat. No. 5,244,726 also discloses a similar admixture of in preparation of inorganic binder where kaolin and fly ash as strengthening agent and sodium laurel sulfate as a surfactant are being used for consolidation of organic and inorganic particulates and fibers in making temperature resistance foamed composites.
Further some other inorganic mineral polymer (geopolymeric) binders of polysialate structures based upon alkaline alumino-silicate compositions have been developed for constructional application. The binder is of alkaline base silico-aluminate polymeric compounds having the characteristics three-dimensional tetrahedral frame works of silicon and aluminium. The tetrahedral are cross-linked by sharing oxygen molecules to form inorganic polymeric network. The alkaline material based upon alumino-silicate polysialate types set for hardening in different conditions such as hydrothermal, elevated temperature and atmospheric temperature. Different workers have explained various methods of preparation of alkaline phases of alumino-silicate polysialate type binders and their applications.
U.S. Pat. Nos. 4,249,386, 4,472,199, 4,509,985 and 5,342,595 Davidovits, et al. reveals the methods for preparation of alumino-silicate polysialate type alkaline mineral binder based upon the reaction of potassium/sodium silicate and hydroxide with the oxides of alumino-silicate (Si2O5,Al2O2)n materials where the aluminum cation is in four fold coordinated state. The binder develops strength by solidification at room and elevated temperature (120° C.). The U.S. Pat. No. 5,352,427 Davidovits, et al. also discloses the preparation of fluoro-alumino-silicate polysialate type hydraulic binder improved in mechanical and heat resistance properties for application in making ceramic products. U.S. Pat. No. 4,888,311, Davidovits describes the use of alkaline alumino-silicate mineral polymer in developing binding matrix by hydrothermal poly-condensation for preparation of composite ceramic materials. U.S. Pat. No. 5,539,140 discloses the method of obtaining alumino-silicate polysialate type termed as geopolymeric binder in powder by the reaction of oxides of alumino-silicate (Si2O5, Al2O2)n materials where the aluminum cation is in four fold coordinated state, disilicates of potassium or sodium and silicates of calcium with improved setting and hardening property.
Preparation of alumino-silicate polysialate type geo-polymeric hardened material has been reported by many workers (1-9). Some of the work defines the dissolution of vitreous components (alumino-silicate glass) of fly ash with alkaline solution in forming polymeric structures of silica and alumina as the binding phase which under temperature develops hardening strength, The effect of composition of non-crystalline glassy structures of coal fly ash on forming geopolymeric binding material have been described (10). It is reported that the geo-polymeric compositions made from fly ash with alumino-silicate glassy phase and calcium bearing alumino-silicate glassy phase when cured above 95% humidity at 23 degree temperature develops 9.5 MPa and 47.5 MPa respectively as the crushing strength. In most cases (11) geo-polymer compositions made from the combinations of fly ash, kaolinite, mica and other alumino-silicate materials are set for hardening in thermal conditions in the temperature ranging from 30° C. to 70° C. The cited references as mentioned below explains to extent the development of alumino-silicate binder based on geopolymeric compositions and its application in making construction materials, ceramics, immobilization of heavy metals and stabilization of mine tailings etc.
Different processes such as firing, hydro-thermal and hydration reactions adopted in manufacture of building construction materials like brick, block and other products using fly ash have been disclosed. These processes explain the formulation of different types of binding matrix that imparts strength and other mechanical properties to the fly ash building products. The firing method develops oxide phases of fly ash binding matrix and the hydro-thermal and hydration method develops hydro-silicate type of binding matrix as the essential feature in developing strength of the fly ash building products. The process adopted on firing at different temperature in manufacture of fly ash building construction products is energy intensive and uneconomical and environmental polluting due to burning of fuel. The other process on formulation of calcium-silicate types of bonding matrix in presence of lime and portland cement and alumino-silicate polysialate type geo-polymeric hardening matrix by hydro-thermal and hydration reactions in producing fly ash building construction products is advantageous over firing processes.
The process of hydro-thermal reaction also requires temperature and steam pressure for curing and hardening which makes the process more complicated and expensive. Further, the process also needs mostly reactive and high pozzolanic character fly ash which makes the process more conducive to use all types of fly ash that generates in the thermal plant. The process of hydro-thermal and hydration reaction based on alumino-silicate polysialate type geo-polymeric binding matrix is also composition specific and also requires mostly glassy structures of alumino-silicate phases of fly ash as the reactant material. Fly ash is used as a partial replacement with other alumino-silicate material to form geo-polymer matrix. Thus it restricts the scope for utilization of different types fly ash available as waste by this process.
In addition the process on formulation of fly ash geo-polymer matrix in manufacturing of building construction materials involve specific quality fly ash, number of processing steps and chemical admixtures, surfactants, catalyzing agents and other source of reactive and costly alumina and silica additives and specific curing and hardening in thermal and humidity conditions not only makes the process complicated but also uneconomical in commercial application. Fly ash, a by-product waste of coal based thermal power plant, is an abundantly available material and a cheap resource of alumina and silica. Thus, adoption of suitable method for complete utilization of alumina and silica of the fly ash in the process of developing binding matrix is advantageous and cost effective for manufacture of fly ash building products. Brick, block and other related construction materials are common building products and their manufacturing process needs to be commercially competitive. Simpler process steps, use of low cost chemical admixture and silica and alumina of fly ash in high volume for commercially competitiveness is of more significant in production of fly ash building products. Thus there exists a need to provide a process that overcomes the aforesaid limitations.